A DC/DC converter is used in electronic equipment and provides voltage and current conversion for a load or a post-stage DC/DC converter, and also prevents the load or the post-stage DC/DC converter from damaging by an abnormal input voltage. Therefore, a response time to transient change of the input voltage is an important technical parameter for the DC/DC converter. According to the conventional art, the DC/DC converter generally utilizes a PWM (Pulse-Width Modulation) controller to achieve the response to the transient change of the input voltage.
A traditional PWM controller, also known as an analog PWM controller, has a feed forward function to response to the transient change of the input voltage. Generally, this function is realized by regulating a slope of a saw-tooth waveform in the PWM controller. FIG. 1 is a block diagram showing a circuit of a traditional PWM controller with feed forward function. As shown in FIG. 1, the traditional PWM controller includes a saw-tooth wave generator, a voltage regulation circuit and a comparator. The saw-tooth wave generator works as a feed forward circuit. The voltage regulation circuit receives an output voltage of the DC/DC converter, and compares the output voltage with a reference voltage so as to generate a voltage Vcomp. Then the voltage Vcomp is compared with a saw-tooth wave voltage by the comparator to generate a duty cycle signal. FIG. 2 shows waveforms in a process of realizing the feed forward function by the traditional PWM controller. The saw-tooth wave voltage Vs rises upward at a starting point of each cycle, such that an output signal will be at a high level. Once the saw-tooth wave voltage Vs exceeds the voltage Vcomp, the output signal will remain a low level until a starting point of a next cycle. The saw-tooth wave voltage is generated by an input voltage Vin through a RC charge circuit, and the slope of the saw-tooth wave voltage depends on the input voltage Vin and any change of the input voltage Vin can directly affect the slope of the saw-tooth wave. Therefore, the duty cycle signal output from the comparator changes with the input voltage Vin. The circuit can quickly respond to the change of the input voltage, but cannot distinguish a stable state from a transient state. If the input voltage Vin keeps high level, the slope of the saw-tooth wave keeps high level and thus a high feedback gain will be generated. In other words, the feed forward circuit cannot be implanted very deeply due to the gain of the stable state. On the other hand, once the identification of the stable state of the input voltage is taken into consideration, the circuit cannot achieve a fast response to the transient change of the input voltage Vin.
With the development of the digital control technology, digital PWM controllers have been widely applied to DC/DC converters due to the advantages of high integration and flexibility. Compared with traditional analog PWM controllers, digital PWM controllers have no need of the saw-tooth wave, and it is not necessary for the comparator to output the duty cycle signals. The duty cycle signals are generated directly by a firmware. However, the conventional digital PWM controllers cannot quickly respond to the transient change of the input voltage Vin. FIG. 3 is a block diagram showing a circuit of a digital PWM controller with feed forward function. As shown in FIG. 3, an independent control loop is introduced in the digital PWM controller. The input voltage Vin is converted into a digital signal through an ADC (Analog to Digital Conversion), and is input to a digital PID controller. A preset algorithm is performed in the digital PID controller, a corresponding value of the duty cycle signal is calculated according to Vin, and combined with a value of the duty cycle signal obtained from an output voltage regulation circuit. The preset algorithm in the digital PID controller may solve the problem of the stable state and transient state, but the response time is too long due to a time delay in ADC and also a program execution time for the firmware and it generally needs dozens of clock cycles to respond to the change of the input voltage.
Furthermore, another problem of the digital controller is that the controller is generally placed at the secondary side of the power converter. The primary side input voltage cannot be directly used as a control signal due to the signal isolation problem. Therefore, the secondary side voltage can only be set as the control signal instead of the input voltage. Although the secondary side voltage is closely related to the input voltage, they are not always identical. For example, in a bridge circuit, the secondary side voltage does not equal to the input voltage because that capacitors at the primary side are used to balance the duty cycle of a primary bridge. In this case, the change of the secondary side voltage fails to absolutely reflect the change of input voltage.